Nowadays high demands are made on the cost effectiveness and the productivity of industrial facilities. On the other hand, there are high demands on the quality of produced work pieces. These contradicting objectives usually can only be achieved with a large degree of automation. Further, several methods for manufacturing are integrated within a single machine tool, in order to reduce the number of cycles and labor. One example is the integration of processes for cutting and grinding in a single processing period.
Known systems for automated thread testing are integrated in an assembly line or provided as independent testing facility. To this end, additional stations or cells for testing are usually integrated in the assembly line to allow for automated testing. Alternatively, manual testing devices may be used for a manual testing of threads.
When measuring the quality using separate measuring cells the work piece has to be discharged from the machine tool and may then automatically be inserted into the measuring cell or random samples of the part are inserted manually into the measuring cell. This requires large investments for additional measuring cells and usually implies a delayed testing. This could lead to a delayed discovery of quality defects and thus to an increased number of defectives. In some approaches the diameter and geometries are measured using inductive sensing elements directly in the machine. In the field of thread testing, however, the testing is carried out afterwards and either automatically or manually.
In general, the quality of internal threads is carried out according to DIN 13 Part 18 as follows: Screwing in the “GO plug gauge” into the full length of the thread by hand should be possible without exerting particular force. If this is not possible, the thread does not meet the requirements. Screwing in the “NO GO plug gauge” more than two revolutions by hand must be impossible from both ends of the thread. If it is possible to carry out more than two revolutions without exerting particular force the thread does not meet the requirements.
The German patent application DE 40 17 376 A relates to means for thread testing, in particular for internal threads.
The German patent application DE 35 34 115 A relates to a self-acting device for assembly and/or testing comprising a measuring support which is attached to an arbor. The displacement of the arbor can be measured using a displacement sensor.
The German patent application DE 43 24 109 A relates to an apparatus for thread testing. It includes a light barrier for sensing the screw in depth of a plug gauge.
The German patent DE 16 23 212 relates to an apparatus for thread testing. The screw in depth can be determined by measuring a relative movement between arbor and guidance using trip cams.
The publication of the German patent application DE 10 2006 034 823 A1 relates to an apparatus and a method for thread testing, wherein a plug gauge is screwed into the thread. Thereby, signals that relate to the screwing in of the plug gauge may be measured and analyzed. These signals may for instance relate to the physical quantities current, active power, torque and/or distance.
However, by screwing in it is only possible to determine whether the diameter of the thread and the thread as such comply with the requirements. This is not sufficient for many processes and additional characteristic quantities are required for a reliable quality inspection. For instance, for safety-relevant parts it is important that the thread is not only tapped correctly, but also that the corresponding screw fits optimal into the thread, in order to yield the best hold.
The technical problem underlying the present invention is therefore to provide an apparatus and corresponding methods for simple and cost-efficient thread testing, wherein more reliable and additional parameters may be measured than in prior art for determining the quality of a thread.